I've not yet tried to scale a Rails site to any serious degree. The only Rails sites I've worked on have a trickle of visitors as they're kind of spare time things. I have found it to be a very productive and tool for getting apps up and running though.

However over the years I have worked on a number of systems that needed to scale to differing degrees and on every one of them the first and most serious bottlenecks I've come across have been nothing to do with the underlying framework or technology choice but due to poor implementation and design based on that framework.

In that sense Rails is just as easy to misuse as anything else.